1. Field of the Invention
The present invention relates to a semiconductor storage device, an operation method of the semiconductor storage device and a test method of the semiconductor storage device.
2. Background Art
A ferroelectric memory is described in Japanese Patent Application Laid-Open No. 9-7378. The ferroelectric memory is a memory which stores a value of binary values associated with the directions of residual dielectric polarization of a ferroelectric substance. Data which can be stored in a single memory cell is only one bit.
A multi-value ferroelectric memory capable of storing a plurality of bits in a single memory cell is described in Japanese Patent Application Laid-Open No. 2003-188350. This ferroelectric memory has a memory cell structure of the so-called 1T (Transistor) type. In the ferroelectric memory, a plurality of bits are stored in a memory cell by changing the residual dielectric polarization quantity of a ferroelectric capacitor by changing a voltage applied to a memory cell at the time of writing and thereby changing the residual dielectric polarization quantity of a ferroelectric capacitor.
In the ferroelectric memory of 1T (one transistor)-type, data remain in the memory cells at the time of readout. In other words, non-destructive readout is possible. In general, however, it is difficult to manufacture such a ferroelectric memory of 1T-type as compared with the 1T-1C (1 Transistor-1 Capacitor) type memory cell structure which is widely advanced in development at the present time.
In the 1T-1C type ferroelectric memory, it is also possible to store a plurality of bits in a single memory cell. For storing a plurality of bits in a single memory cell, however, a plurality of write voltages are needed at the time of data writing. In addition, in the 1T-1C type ferroelectric memory, data does not remain in the memory cells. In other words, the data may be destroyed at the time of readout. At the time of readout, therefore, it is necessary to rewrite the same data in the memory cells. At the time of the data rewriting as well, a plurality of write voltages are needed.
Thus, unlike the flash memory, it is difficult in the ferroelectric memory to increase the degree of integration by using the technique of storing a plurality of bits in a single memory cell and provide an inexpensive, large-capacity memory.
A semiconductor storage device is desired, which has a ferroelectric memory that is highly integrated, inexpensive, a large capacity ferroelectric memory by storing a plurality of bits in a single memory cell.